There is quite some information around about making one box a dedicated rsync server so you can run emerge sync within your LAN. This is related to, but not quite identical to my question: my G5 is a double-boot, and I use portage in both OS X and linux. Could I simply run "emerge sync" in one of the systems and then do the syncing internally, like so:

Not sure if i understand your question correctly.
I would mount the osx filesystem under linux and symlink to the /usr/portage dir in the osx partition.
Have to warn you though... I dont have acces to a mac atm and i dont know much the portage-on-mac program._________________I laugh in the face of danger... Then i hide until it goes away.

Yes, that was my question - does the difference in architecture make any difference for the /usr/portage directory, or are they identical? Then either a symlink could do the job, or I could see to it that /usr/portage on both systems be identical. Thanks, I will just try and see!

Yup, I tried, and it seems to be working. Instead of having a /usr/portage on my OS X partition, I now have a symlink pointing to my gentoo partition. And oddly enough, I have the impression that emerge now is much faster than before! Anyway, thanks for the help, I'm glad I have to sync only once now!